Pneumatic rubber tires are often desired to have treads of a rubber composition which will provide properties which include good traction on the road, resistance to tread wear and low rolling resistance. It has traditionally been difficult to improve one of such properties without sacrificing one or more of the other properties. Such properties depend, to a great extent, upon dynamic viscoelastic properties of elastomers used in making the tread rubber composition as well as viscoelastic properties of the rubber composition itself.
For example, elastomers, or rubber composition, presenting a higher rebound property can generally promote lower rolling resistance for a tire with a tread containing such elastomer(s) or rubber composition. For example, elastomers, or rubber composition, presenting a higher resistance to abrasion might generally promote greater resistance to tread wear for a tire tread containing such elastomer(s) or rubber composition. For example, elastomers, or rubber composition, having a higher glass transition temperature (Tg) may promote greater traction, or skid resistance, for as tire tread containing such elastomer(s) or rubber composition.
A challenge is presented to balance these three viscoelastic properties, which can be largely inconsistence properties in a sense that improving one of such properties may sacrifice one or more of the other two properties. The challenge is to evaluate specified combination of elastomers sometimes used in tire treads to achieve a beneficial balance of properties.
Traction emphasizing tread rubber typically exhibits a relatively high, single, glass transition temperature (Tg) of above −50° C. and usually within a range of from about zero to about −50° C.
For this evaluation, a rubber composition is evaluated which is comprised of a silica-rich filler reinforcement containing rubber composition comprised of a combination of elastomers of cooperative viscoelastic properties comprised of specialized polybutadiene rubber, functionalized styrene/butadiene rubber and cis 1,4-polyisoprene rubber.
In the description of this invention, terms such as “compounded rubber”, “rubber compound” and “compound”, if used herein, refer to rubber compositions composed of one or more elastomers blended with various ingredients, including curatives such as sulfur and cure accelerators. The terms “elastomer” and “rubber” might be used herein interchangeably. It is believed that all of such terms are well known to those having skill in such art.
A reference to glass transition temperature, or Tg, of an elastomer or elastomer composition, where referred to herein, represents an inflection point glass transition temperature(s) of the respective elastomer or elastomer composition in its uncured state or possibly a cured state in a case of an elastomer composition. A Tg can be suitably determined by a dynamic mechanical analyzer (DMA), an RSA III instrument from TA Instruments, which would be recognized by those having skill in such art, as well as, for example, a differential scanning calorimeter (DSC), at a temperature rate of increase of 10° C. per minute.
A resin's softening point (Sp) may be determined by ASTM D-28.
For this invention, the term “functionalized” relates to elastomers which contain at least one functional group which is reactive with hydroxyl groups (e.g. silanol groups) contained on precipitated silica reinforcement for the rubber composition.